The present invention is a technique relating to a surface light emitting device used for a liquid crystal display and others.
As a device for illuminating a liquid crystal display board and others, a surface light emitting device referred to as a front light or back light has been proposed. In the surface light emitting device, a point light source or a line light source is arranged on the side of a light-guide plate, a beam of light emitted by the light source enters into the light-guide plate and reflected, and the beam of light emits upward or downward with respect to the light-guide plate. In order to realize the emitting of light described above, irregularities for reflecting light sent from the light source are formed on a predetermined face of the light-guide plate or on an optical sheet laminated on the light-guide plate. Concerning the forms of providing these irregularities, various forms are proposed so as to enhance the luminance and reduce the unevenness of the luminance.
The forms of these irregularities are divided into two main forms. One is a form in which the irregularities are made into points of various shapes such as a column, circular cone and hemisphere, and the other is a form in which the irregularities are made into various linear shapes, the cross sections of which are formed into a triangle and others. In the former form in which the irregularities are made into points of various shapes, the diffusion of light tends to be facilitated, and the unevenness of luminance tends to be reduced. However, the luminance tends to be deteriorated. On the other hand, in the latter form in which the irregularities are made into various linear shapes, the luminance tends to be enhanced, however, the unevenness of luminance tends to be generated along the lines.
As a specific example of the related art, for example, Patent Document 1 discloses a technique in which a projecting portion having a pair of inclined faces formed In a direction perpendicular to an entrance face is repeatedly formed in a direction perpendicular to the entrance face on a reflection face opposed to an emitting face. Patent Document 2 discloses a technique in which the same projecting portions are provided on the emitting face.    Patent Document 1: JP-A-10-268138    Patent Document 2; JP-A-2003-86012
In the case where a surface light emitting device is incorporated into a small device such as a cellular phone, a sufficiently large space can not be provide for arranging the light source. Therefore, a point light source such as LED is preferably adopted, compared with a line light source such as a cold cathode ray tube. However, in the structure disclosed in Patent Documents 1 and 2, the following problems may be encountered. Although the emitting efficiency is high, in the case of using a point light source, a front portion of the light source with respect to a direction perpendicular to the entrance face becomes bright linearly, and further unevenness of the luminance tends to be caused in the vicinity of the light source on the emitting face.